Its called a DCDU - Data Communication Display Unit, you can find them on anything from the 320 series to 380. Its for CPDLC (Controller Pilot Data Link Communications), the pilots "Log On" to the ATC centre and instructions to and from the aircraft are passed to the aircraft via CPDLC instead of voice communications on VHF or HF
The flightcrew is alerted to an incoming message by means of a flashing blue «ATC MSG» light in two pushbuttons on the glareshield, as well as by a dedicated audio sound which is similar to a telephone sound. The alert is stopped by pressing one of these two pushbuttons or by answering the message, directly on the DCDU. For normal messages the buttons flash, and the audio signal is repeated about every 15 seconds (with the first signal delayed by 15 seconds, so as not to multiply audio warnings). The message will appear on the DCDU if the screen is empty. If the screen is not empty, a flashing cue (e.g. «MSG 1/2») reminds the crew of the arrival of the message. For urgent messages the buttons flash, the audio signal is repeated about every 5 seconds, and the message is displayed on the DCDU regardless of the state of the screen.
To reply to a message, the flightcrew either uses the standard replies on the DCDU (WILCO, UNABLE, STANBY, ROGER, AFFIRM, NEGATIVE etc) or composes a reply on a menu-page from the MCDU. After composing the message on the MCDU it is transferred to the DCDU for sending.
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